'Existentialism' is a very broad term. It can include atheists (Heidegger, Camus, Sartre, certain interpretations of Nietzsche) and theists (Karl Jaspers, Søren Kierkegaard); it can be humanist (Sartre's '
Existentialism Is A Humanism' lectures) or antihumanist (Heidegger's '
Letter On Humanism', written partially as a rebuttal to Sartre's lecture); politically conservative like Heidegger or outright Marxist like Franz Fanon. It can even be idealistic in the philosophical sense - Emmanuel Levinas and most of Heidegger's phenomenological followers, who reduced existence to consciousness - or materialistic, like Marxist existentialists. Many of those associated with the movement, like Heidegger and Camus, disavowed the term.
Existentialism is, then, so broad that it's nearly meaningless (this is, I suppose, ironic).
But I think there are certain tendencies that are definably existentialist, certain themes that, when taken together, deserve to be classified as a movement in itself.
1. All of the 'existentialist' authors focused on
action as
a priori to knowledge; if the European philosophical tradition began with the Cartesian reduction of the Self to thought, the existentialists suggested that human beings
do things before they
are things. You have to eat, after all, before you can think.
2. All or virtually all of them, religious or otherwise, denied that the world was immediately comprehensible to human beings; the atheists rejected vulgar scientific positivism; the Christians denied that God's plan was clear and knowable to any who would just believe.
3. Existentialism in all of its forms sought to analyze the contents of consciousness without resorting to causal chains; that is, owing to its roots in Husserlian phenomenology, they sought to look at the categories of human thought without reference to externalities.